Kinetic, gravitational potential and elastic potential energy equations, specific heat capacity with its required practical, and calculating power.
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1.State the equation for kinetic energy.
kinetic energy = 0.5 x mass x speed squared
2.What are the units in the kinetic energy equation?
Energy in joules, mass in kilograms, speed in metres per second.
3.Calculate the kinetic energy of a 2 kg object moving at 10 m/s.
0.5 x 2 x 100 = 100 J.
4.What happens to kinetic energy if speed doubles?
It increases by a factor of four, because speed is squared.
5.What happens to kinetic energy if mass doubles?
It doubles, because energy is directly proportional to mass.
6.State the equation for gravitational potential energy.
gravitational potential energy = mass x gravitational field strength x height
7.What is the value of gravitational field strength on Earth?
About 9.8 newtons per kilogram, often taken as 10 in calculations.
8.Calculate the gravitational potential energy gained by a 5 kg mass lifted 3 m. Use g = 10.
5 x 10 x 3 = 150 J.
9.State the equation for elastic potential energy.
elastic potential energy = 0.5 x spring constant x extension squared
10.What assumption is made when using that equation?
That the limit of proportionality has not been exceeded.
11.A spring with constant 200 N/m is extended 0.1 m. Calculate the energy stored.
0.5 x 200 x 0.01 = 1 J.
12.State the equation for the energy needed to change temperature.
change in thermal energy = mass x specific heat capacity x temperature change
13.What is specific heat capacity?
The energy needed to raise the temperature of one kilogram of a substance by one degree Celsius.
14.What is the unit of specific heat capacity?
Joules per kilogram per degree Celsius.
15.Calculate the energy to heat 2 kg of water by 10 C. Specific heat capacity is 4200.
2 x 4200 x 10 = 84000 J.
16.What does a high specific heat capacity mean?
A lot of energy is needed to change the temperature, so the substance heats up and cools down slowly.
17.Why is water used in central heating systems?
It has a very high specific heat capacity, so it can carry a lot of energy for each degree it cools.
18.In the specific heat capacity practical, what is measured?
The mass of the block, the energy supplied by the heater, and the temperature rise.
19.Why is the block insulated in that practical?
To reduce energy transferred to the surroundings, which would make the measured specific heat capacity too high.
20.Why is oil put in the hole for the thermometer?
To improve thermal contact between the block and the thermometer, so the reading is accurate.
21.State the equation for power in terms of energy.
power = energy transferred divided by time
22.State the equation for power in terms of work.
power = work done divided by time
23.What is the unit of power?
The watt, W. One watt is one joule per second.
24.A device transfers 3000 J in 60 s. Calculate its power.
3000 divided by 60 = 50 W.
25.Two motors do the same work, one in half the time. Compare their power.
The faster one has twice the power, because it transfers the same energy in half the time.
26.Calculate the energy transferred by a 2000 W kettle in 90 s.
2000 x 90 = 180000 J.
27.An object falls. Which equation links its potential and kinetic energy?
Ignoring air resistance, the gravitational potential energy lost equals the kinetic energy gained, so mgh = 0.5mv squared.
28.A 1 kg ball falls 5 m. Calculate its speed on landing. Use g = 10.
Potential energy = 1 x 10 x 5 = 50 J. So 0.5 x 1 x v squared = 50, v squared = 100, v = 10 m/s.
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